1. Field of the Invention
The present invention relates to a hydraulic transmission coupling apparatus using generated hydraulic pressure of a hydraulic pump, especially a vane pump, as a transmitting means for a driving force existing between front and rear wheels.
2. Description of Prior Art
As a four-wheel drive vehicle is superior in running stability not only on a specific surface such as a snow-covered road and a gravel road, but also on a usual road even at a time of accelerating or decelerating and of high speed running, it has especially recently been spotlighted for its comfortable running without being affected by the road condition and running state. The full time four-wheel drive vehicle having, on the way of a transmission shaft between front and rear wheels, a transmission coupling apparatus which distributes the driving force corresponding to a rotation speed difference generated between the front and rear wheels, prevents the so-called tight corner breaking phenomenon and is a leading figure in the four-wheel drive vehicle of this kind. The tight corner breaking phenomenon is the one in which a rotation speed difference can not be absorbed, thereby the inside rear wheel being dragged in a condition of slipping, in the case where the rotation speed difference is created between the front and rear wheels due to the turning radial difference between the front and rear wheels at the time of turning. When the tight corner breaking phenomenon is created, there is a fear that it not only brings about bad steering feeling, but leads a propeller shaft between the front and rear wheels to be twisted or causes abnormal wearing at a tire.
As a transmission coupling apparatus for preventing aforesaid tight corner breaking phenomenon, there is a hydraulic transmission coupling apparatus using a hydraulic pump, especially a vane pump which is easily small-sized and light-weighted, and is superior in durability.
A vane pump is well-known, being comprised of a rotor and a casing. The rotor of a short cylindrical body has a plurality of grooves, each of which is of a predetermined depth in the radial direction thereof in almost the same interval from the next in the peripheral direction. A plate-like vane is slidably interposed to each groove. The casing is constructed by installing side plates on both sides of an annular cam ring of wall eccentricity, having space surrounded by these cam ring and side plates inside thereof. In the inside space of aforesaid casing, aforesaid rotor is coaxially and rotatably stored. In a space between the outer circumferential surface of aforesaid rotor and the inner circumferential surface of the cam ring, a plurality of pump chambers having a crescent section, surrounded by these rotor and cam ring and aforesaid side plates, are formed. The vane pump is so constructed as to pressurize hydraulic fluid which has been drawn from a suction port positioned at an end portion of the crescent portion of each pump chamber by rotating the hydraulic fluid being in the condition of being confined between the adjoining vanes corresponding to the rotation of the rotor, and to discharge it from a discharge port positioned at the other end portion of the crescent portion. In the hydraulic transmission coupling apparatus using vane pumps of this kind, aforesaid rotor is fixed to the shaft connected to one of the front and rear wheels and aforesaid casing to the shaft connected to the other respectively and coaxially. The hydraulic transmission coupling apparatus is designed to create, between the rotor and the casing, relative rotation corresponding to the rotation speed difference between the front and rear wheels, to generate the hydraulic pressure corresponding to the relative rotation at each of aforesaid pump chambers, and to transmit the driving force between the rotor and the casing by the resistance force created according to the hydraulic pressure. In such a hydraulic transmission coupling apparatus, as the casing rotates interlockingly with one of the front and rear wheels, that is, it is not fixedly installed, the hydraulic fluid of the vane pump is, as is disclosed in Japanese Patent Application Laid-Open No. 1-250625 by the inventors, confined in an annular hydraulic tank formed between a cylindrical surrounding member of wall eccentricity installed on the casing in the condition of surrounding the outside of the casing and the outer periphery of aforesaid casing. The hydraulic fluid is drawn to each of aforesaid pump chambers through a suction oil passage which opens at the outer periphery of the side plate and communicates with aforesaid suction port. And also as disclosed in aforesaid Laid-Open Application, each vane has a throttle hole of a small diameter which passes through both sides of the vane. A part of hydraulic fluid confined between the adjoining vanes is made to be leaked from high pressure side to low pressure side by passing through the throttle holes so as to obtain generated pressure corresponding to the conduction resistance at this time. In addition, discharged hydraulic fluid from the discharge port is drawn to a bottom part of a storage groove of each vane through a communication groove formed annularly at the side of the rotor, this hydraulic pressure pressuring each vane in the outward radial direction, thereby a head of each vane is surely slidably fitting the inner circumferential surface of the cam ring.
Next, in the transmission coupling apparatus used for a four-wheel drive vehicle, it is important for transmission torque to show proper transmission characteristic against the increase or decrease of the rotation speed difference generated between the front and rear wheels. The proper transmission characteristic is the one which makes the transmission torque smaller in the case where the rotation speed difference is small as in the turning or the like, and makes the transmission torque larger in the case where the rotation speed difference is large as in the running on snow-covered road or running on upward slope. Thereby, in the case where the rotation speed difference is small, the rotation speed difference can be absorbed by making the transmission torque smaller, and the tight corner breaking phenomenon is prevented. In the case where the rotation speed difference is large, the driving force is transmitted to the four wheels reliably by making the transmission torque larger, leading to stable running in these running conditions.
Therefore, in the hydraulic transmission coupling apparatus employing a vane pump, it is important for the pressure in the pump chamber to show proper transmission characteristic against the increase or decrease of the relative rotation speed between the rotor and the casing.
In the conventional transmission coupling apparatus constructed as above-mentioned, the increase or decrease characteristic of aforesaid pressure depends upon the flow resistance at the throttle hole formed at each vane, and upon the conduction resistance created when the hydraulic fluid drawn to the bottom part of the storage groove, in aforesaid way, leaks from the gap between the side surface of the side plate and that of the rotor to the low pressure portion. For this reason, in order to realize the proper characteristic, high accuracy in processing a throttle hole at each of a plurality of vanes is required. Therefore, it is a problem that the processing cost of vanes is very expensive.
In addition, the hydraulic fluid of the vane pump is confined inside the fluid tank of an aforesaid construction, whose size having a limit, and stirred by the rotation of the rotor. Therefore, in the case where large rotation speed difference is created between the front and rear wheels at the time of running on snow-covered road or upward slope, thereby the running state requiring high transmission torque continues for a long time, the temperature of hydraulic fluid rises as it is continued to be stirred, resulting in lowering of the viscosity of the hydraulic fluid inevitably. The transmission of the driving force between the front and rear wheels, as afore described, is carried out by resistance force generated between the rotor and the casing, corresponding to the pressure of the inside of the pump room. The generated pressure inside of the pump room depends upon the flow resistance of the throttle hole provided at the vane and the flow resistance of the gap between the rotor and the side plate. Accordingly, the generated pressure based on the same rotation speed difference decreases as the viscosity lowers. In the case where the same generated pressure is obtained, aforesaid resistance generated by the rotation speed difference decreases as the viscosity lowers. For this reason, the temperature rise of the hydraulic fluid brings about the lowering of transmission torque based on the same rotation speed difference. The transmission characteristic of transmission torque against the rotation speed difference, as shown in FIG. 1, equally decreases as the temperature of the hydraulic fluid rises. Especially in the case where the rotation speed difference is large, transmission torque is largely decreased. Therefore, in order to obtain the same transmission torque at the time of high temperature as the one at the time of low temperature, the rotation speed difference larger than that at the time of low temperature is required.
In such a conventional apparatus, in the case where the high transmission torque is to be obtained, a large rotation speed difference is required. But it is a problem that the hydraulic fluid is heated by the large rotation speed difference, thereby the desired transmission of torque cannot be carried out.